John,
They are referring to MUTCD, Manual on Uniform Traffic Control Devices. You can read it here.
Specifically, they are talking about their Holy Grail, setting speed limits to the 85th Percentile. For this we go to Section 2B.13 Speed Limit Sign.
I'll qute the relevant pbuttage here for you:
Standard: After an engineering study has been made inaccordance with established traffic engineering practices, the Speed Limit (R2-1) sign (see Figure 2B-1) shall display the limit established by law, ordinance, regulation, or as adopted by the authorized agency. The speed limits shown shall be in multiples of 10 km-h or 5 mph.
Guidance: At least once every 5 years, States and local agencies should reevaluate non-statutory speed limits on segments of their roadways that have undergone a significant change in roadway characteristics or surrounding land use since the last review.
No more than three speed limits should be displayed on any one Speed Limit sign or buttembly.
When a speed limit is to be posted, it should be within 10 km-h or 5 mph of the 85th-percentile speed of free-flowing traffic.
Option: Other factors that may be considered when establishing speed limits are the following:
1. Road characteristics, shoulder condition, grade, alignment, and sight distance; 2. The pace speed; 3. Roadside development and environment; 4. Parking practices and pedestrian activity; and 5. Reported crash experience for at least a 12-month period.
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End quoted material.
See, it right there in black and white. The speed limit must be set to the 85th Percentile.
Doug
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